Polyacetal polymers, which are commonly referred to as polyoxymethylenes (POMs), have become established as exceptionally useful engineering materials in a variety of applications. POMs for instance, are widely used in constructing molded parts, such as parts for use in the automotive industry and the electrical industry. POMs, for instance, have excellent mechanical property, fatigue resistance, abrasion resistance, chemical resistance, and moldability.
Although polyacetal resins possess many useful properties, the polymers have a tendency to degrade when heated and are inherently unstable in an oxidative atmosphere or in an acidic or alkaline environment. In particular, polyacetal resins have a tendency to emit formaldehyde during processing and after the polymer has been molded into a part. Formaldehyde is not only a contaminant, but can also adversely affect metallic components that may be placed in association with the polymer. For example, formaldehyde readily oxidizes to formic acid which can corrode metals or cause discoloration.
In view of the above, those skilled in the art have suggested using various stabilizers in molding compositions containing polyacetal polymers in order to suppress degradation. For example, various stabilizers are disclosed in JP-08208946, in Japanese Patent Application Kokai No. 60-90,250, in Japanese Patent Application Kokai No. 7-173,368 and in Japanese Patent Application No. 7-331,028. Various stabilizers that have been proposed in the past include, for instance, nitrogen containing compounds such as a dicyandiamide or an amino-substituted triazine compound.
Unfortunately, however, the above stabilizers have a tendency to bleed or migrate to the surface of molded articles, thereby forming surface imperfections. Thus, a balance typically has to be established between reducing formaldehyde emission and in producing an article with desired properties. In U.S. Pat. No. 6,051,660, for instance, controlled amounts of amino-substituted triazines are added to polyacetal resins containing from 1000 to 3000 ppm of formaldehyde.
Problems associated with maintaining the properties of the polymer in conjunction with reducing formaldehyde emission can become exacerbated when attempting to incorporate a colorant into the polymer mixture. For example, in the past, only a limited number of colorants have been suggested for use in pigmenting polyacetal polymers without adverse effect on the properties of the material and/or the product.
For example, U.S. Pat. No. 6,306,940 is directed to colorant concentrates for dyeing polyoxymethylene molding materials. The above patent teaches the use of a colorant in conjunction with a nitrogen-containing stabilizer and a dispersion aid for producing colored injection moldings or extrudates that are particularly stable during processing and have low formaldehyde emissions.
U.S. Pat. No. 7,186,766 is directed to the use of a phenol component and/or an amino acid in conjunction with a polyacetal resin with or without a colorant in order to inhibit formaldehyde emission.
U.S. Patent Application Publication No. 2008/0287580 teaches that adding colorants to polyoxymethylene polymers can lead to the breakdown of the material during processing with subsequent release of formaldehyde. In this regard, the above patent application teaches the use of a colorant in combination with a nitrogen-containing stabilizer, an ester of a polyhydric alcohol and at least one fatty acid, and a metal salt of a fatty acid in polyoxymethylene molding compositions in order to increase stability, lower formaldehyde emission, produce defect-free surfaces, and produce high colorfastness.
U.S. Pat. No. 7,449,239 is directed to coated metallic luster pigments for pigmenting high molecular weight materials. The '239 patent discloses coating a metal platelet, such as aluminum flakes with a low refractive dielectric layer which does not absorb visible light for pigmenting the polymer materials.
Although the above identified patents and patent applications have provided some advances in the art, a need still exists for an improved molding composition containing a polyacetal resin that has reduced formaldehyde emissions. In addition, a need also exists for a method of incorporating a colorant or pigment into a polyacetal polymer that does not increase formaldehyde emission levels or adversely affect the physical properties of the polymer, such as by causing surface defects.